Make the Buckwheat Dough:

• In a food processor, pulse together the all-purpose flour, buckwheat flour, and salt.
• Add the cold butter and oregano, then pulse until the mixture resembles coarse crumbs.
• With the processor running, drizzle in the cold water until the dough starts to pull away from the sides.
• Gather the dough into a flat disk, wrap in plastic wrap, and chill for 45 minutes (or up to 3 days).

Prepare the Vegetables:

• Heat a 12” skillet over medium heat and coat the bottom with olive oil.
• Add the fennel with a pinch of salt and cook for 6-8 minutes, stirring often, until soft and caramelized. Transfer to a plate.
• Add a little more olive oil, then sauté the leeks with a pinch of salt for about 2 minutes. Transfer to the plate.
• Add a bit more olive oil, then cook the chard with a pinch of salt and the juice of half a lemon for 30 seconds, until just wilted.

Assemble the Crostata:

• Preheat the oven to 425°F.
• Let the chilled dough sit at room temperature for a few minutes, then roll it out on a lightly floured surface into a 12” circle.
• Transfer to a parchment-lined sheet pan.
• Sprinkle half the cheddar over the dough, leaving a 1” border.
• Layer the chard, fennel, leeks, and remaining cheddar on top.
• Fold the edges of the dough over the filling, pressing any cracks together.
• Brush the edges with the beaten egg and sprinkle with 1 tbsp sesame seeds.

Bake and Serve:

• Bake for 35 minutes, until the crust is golden and crisp.
• In a small bowl, mix the honey, oregano, 2 tsp lemon juice, and remaining 2 tsp sesame seeds.
• Slice the crostata into 6-8 pieces.
• Drizzle with the honey mixture and sprinkle with flaky sea salt before serving.